RESPIKATOKY SYSTEM OF INSECTS. 



321 



these openings, ten in number on each side of the body, are situated 

 ^n the soft membrane interposed between the different rings, and not 

 in the rings themselves, a provision for the purpose of allowing their 

 orifices to be opened or closed at pleasure, instead of being rigid and 

 motionless. The margin of the spiracle is frequently encompassed by 



Tig. 161. 



thick horny lips, which 

 may be approximated 

 by muscles provided for 

 the purpose, so that the 

 opening can be shut at 

 pleasure, in order to 

 exclude any extraneous 

 substances that might 

 otherwise obtain admis- 

 sion. In many insects, 

 indeed, especially in bee- 

 tles which crawl upon 

 the dusty ground, an 

 additional provision is 

 necessary to prevent the 

 entrance of foreign mat- 

 ter; and in such cases 

 the spiracles are seen to 

 be covered with a dense 

 investment of minute 

 and stiff hairs, so dis- 

 posed as to form a sieve 

 of exquisite fineness, 

 a beautiful contrivance 

 by which the air is fil- 

 tered, as it were, before 

 it is allowed to pass into 

 the breathing-tubes, and 

 thus freed from all pre- 

 judicial particles. From 

 every spiracle is derived 

 a set of extremely deli- 

 cate tubes (trachece),tha,t 

 pass internally, and be- 

 come divided and sub- ~~ - : ---~ 



divided to an indefinite Respiratory apparatus of Melolonthavulgaris. 



extent, penetrating to every part of the body, and ramifying through 

 all the viscera ; so that air is thus supplied to the entire system. Upon 

 more minutely inspecting these air-tubes, they are found to assume 

 various forms in different parts of the body, being sometimes simple 



